1. Field of the Invention
Embodiments of the present invention generally relate to semiconductor device packaging and, more particularly, to semiconductor packaging for heat dissipation away from the semiconductor die(s).
2. Description of the Related Art
Heat transfer management is a concern for designers of semiconductor devices from simple devices, such as light-emitting diodes (LEDs), to more complex devices, such as central processing units (CPUs). When such devices are driven with high currents, high device temperatures may occur because of insufficient heat transfer from the p-n junctions of the semiconductor die to the ambient environment. Such high temperatures may harm the semiconductor and lead to such degradations as accelerated aging, separation of the die from the lead frame, and breakage of bond wires.
For an LED, in addition to the aforementioned problems, the optical properties of the LED vary with temperature, as well. As an example, the light output of an LED typically decreases with increased junction temperature. Also, the emitted wavelength can change with temperature due to a change in the semiconductor bandgap energy.
The main path for heat dissipation (thermal path) in semiconductor devices encased in ceramic packages 110 (e.g., low temperature cofired ceramic (LTCC) or alumina) of the prior art, as shown in FIG. 1, is from the p-n junctions of one or more semiconductor dies 120 to the lead frame 130 via bond wires 140 and then through the ends of the leads (i.e., the terminals 150) via heat conduction. At the terminals 150 heat conduction, convection, and radiation serve to transfer heat away from the semiconductor device when mounted on a printed circuit board (PCB), for example. There is also a secondary path of heat conduction from the surfaces of the semiconductor dies 120 to surfaces of the ceramic package 110 or, in some cases, encapsulation materials 160.
One problem with this design described thus far is that the majority of the lead frame 130 is situated within the ceramic package 110, which acts as a thermal insulator, and the main path for heat dissipation out of the device is limited by the size of the leads. Even designs that have added to the size or number of leads in an effort to promote heat transfer still possess an inherent bottleneck for heat dissipation, as the leads are still sandwiched in the thermally insulative ceramic package 110.
To mitigate this bottleneck, designers have added electrically conductive thermal vias 170 disposed in a bottom portion of the ceramic package underneath the semiconductor dies 120 in an effort to provide improved heat dissipation away from the dies 120 through the growth substrate (e.g., silicon, sapphire, silicon carbide, and gallium arsenide) and the thermal vias 170 to a printed circuit board (PCB), heat sink, or other suitable entity on which the electronic device is mounted. However, the insulative properties of the growth substrate and the ceramic package 110 surrounding the thermal vias 170 limit the potential heat transfer.
Another conventional thermally conductive package 200 for electronic components is illustrated in FIG. 2. This package 200 consists of a plurality of electrically insulative layers 210 of glass or ceramic bonded to a metal base plate 220. The semiconductor die(s) 230 are mounted on the base plate 220, and then bond wires 240 are used to connect the contact pads 250 of the die(s) 230 to vias 260. Because the base plate 220 is bonded to the bottom of the stack of insulative layers 210, terminals 270 for mounting to a PCB may be located at the top of the package 200.
To form the package 200, layers of green tape composed of glass particles in a binder are formed. Openings or holes may be punched or cut to provide for any desired openings (e.g., an opening for mounting the semiconductor die 230) or vias 260 in the package. The layers of green tapes are then stacked on each other and on the surface of the base plate 220. This assembly is then fired at a temperature which drives off the binder and melts the glass particles to form a glass or ceramic body 280. In this process, the temperature is restricted to the metal materials of the base plate. In other words, the coefficient of thermal expansion (CTE) should be similar between the metals of the base plate 220 and the green tape layers of the body 280, and the firing temperature must be less than 1000° C.
Accordingly, what is needed is a technique to packaging semiconductor devices that increases heat dissipation and simplifies the manufacturing process when compared to conventional packaging techniques, preferably while allowing for higher firing temperatures.